If you cut a testicle in half, you’d see a dense white outer shell surrounding hundreds of tightly packed compartments, each filled with coiled, thread-like tubes. The overall internal tissue appears brownish and has a soft, somewhat spongy texture. It looks nothing like a uniform mass. Instead, it’s a highly organized organ with distinct layers, internal walls, and a surprisingly complex plumbing system.
The Outer Shell and Internal Walls
The first thing you’d notice on a cross-section is the outer covering, called the tunica albuginea. It’s a dense, white, inelastic layer of fibrous tissue that gives the testicle its firm feel when you press on it from the outside. This shell is thick enough to see clearly with the naked eye, and it’s what gives the cut surface its distinctive white border.
From this outer shell, thin walls of tissue extend inward like the spokes of a wheel, dividing the interior into roughly 250 separate compartments called lobules. These internal walls all converge toward the back of the testicle at a thickened ridge of tissue called the mediastinum, which acts as the central hub where fluids and sperm collect before exiting. On an ultrasound, the mediastinum shows up as a bright, thin band running along the back of the organ, and the outer shell appears as a bright line outlining the entire testicle.
What Fills Each Compartment
Each of those 250 lobules is packed with tightly coiled tubes called seminiferous tubules. These are where sperm are actually produced. A single testicle contains between 800 and 1,600 of these tubules, and if you could uncoil and stretch them all end to end, they’d reach about 600 meters, longer than the Empire State Building is tall. That enormous length is crammed into an organ that only measures about 5 centimeters long and 3 centimeters tall in most adults, weighing around 20 grams.
Under a microscope, each tubule has a distinct circular cross-section with layers of developing sperm cells lining the inner wall. The cells at the outer edge are immature, and as they divide and mature, they migrate inward toward the hollow center of the tube. By the time they reach the middle, they’re recognizable as sperm with tails. Scattered between these developing sperm cells are larger support cells that serve as scaffolding and nourishment, feeding the maturing sperm and controlling the local chemical environment.
In the spaces between the tubules sits loose connective tissue containing clusters of hormone-producing cells. These cells manufacture testosterone, which then diffuses into the nearby tubules to drive sperm production. Together, the support cells inside the tubules, the hormone-producing cells outside them, and the developing sperm cells form a tightly coordinated system that handles both reproduction and hormone regulation simultaneously.
The Internal Plumbing Network
Sperm don’t just float freely inside the testicle. They follow a specific route. From the coiled seminiferous tubules, sperm flow into short, straight connecting tubes that empty into a mesh-like network of channels nestled inside the mediastinum at the back of the organ. This network looks like a fine web of interconnected passages, and it serves as a collection basin.
From there, sperm exit the testicle through a series of small ducts that bridge the gap between the testicle itself and the epididymis, the comma-shaped structure that sits along the back and top of each testicle. In humans, the branching pattern of these exit ducts is particularly complex, with multiple channels connecting to the main epididymal duct. The epididymis is where sperm finish maturing and are stored until ejaculation.
Blood Supply and Temperature Control
The interior of the testicle has a rich blood supply, with the main artery entering through the mediastinum and branching out through the tissue between lobules. What makes this blood supply unusual is how it handles temperature. Sperm production requires temperatures about 2 to 4 degrees Celsius below core body temperature, and the body has a built-in cooling system to make that happen.
The veins draining the testicle form an intricate web called the pampiniform plexus that wraps around the incoming artery within the spermatic cord. This arrangement works as a countercurrent heat exchanger: the cooler venous blood absorbs heat from the warmer arterial blood before it reaches the testicle, pre-cooling it. If this system is disrupted, for instance by abnormal connections between the vessels, the resulting temperature increase can impair sperm production.
A Hidden Immune Barrier
One of the most remarkable features inside the testicle is invisible to the naked eye. The support cells lining the seminiferous tubules form an extremely tight seal with one another, creating a barrier that prevents immune cells and most molecules in the bloodstream from reaching developing sperm. This matters because sperm cells don’t appear in the body until puberty, long after the immune system has learned to recognize the body’s own tissues. Without this barrier, the immune system would treat sperm as foreign invaders and attack them. The seal is so effective that even tiny tracer molecules used in laboratory experiments cannot pass through it.
How It Looks on Imaging
Most people will never see a testicle cut open, but many will see their testicle’s interior on an ultrasound. On a standard scrotal ultrasound, healthy testicular tissue appears as a uniform, medium-gray texture throughout. The outer shell shows up as a thin bright line, and the mediastinum appears as a bright band along the back. In about one in five patients, the sperm-collecting network near the mediastinum is visible as a slightly darker area. Any disruption in that uniform gray pattern, such as a dark spot or irregular texture, is what prompts further evaluation.
Normal testicular volume on ultrasound falls between 15 and 25 milliliters. A measurement below 12 milliliters is considered low and can be associated with reduced sperm production, since the vast majority of testicular volume comes from those densely packed seminiferous tubules. In other words, a smaller testicle generally means fewer or shorter sperm-producing tubes inside.